void x(void);
void perm(char * a, int n){
  // This routine permutes the n characters starting at a and for
  // each permutation, calls x.
  // The characters are left as they were found.
  if (n < 2) x();
  else {char t = *a; int j=n; while(j--) {
      char u = *(a+j); *a = u; *(a+j) = t; perm(a+1, n-1); *(a+j) = u;}
    *a = t;}}

// Demo and test:
#include <stdio.h>
#include <string.h>
char q[8];
int main () {strcpy(q, "abcdefg");
perm(q, 7); return 0;}
void x() {static int c=0;
  if(!(c--)) {printf("\n"); c=6;}
  printf("%s ", q);}